In a matter of hours, lucky observers with clear skies will be able to watch Venus pass in front of the Sun. Transits of Venus are rare – this is the last one until 2117 – but that’s not the only reason you should find a way to watch it. This astronomical event is historically very significant. Since the 17th century astronomers have used Venus transits to better understand the Universe and our place within in, and the upcoming transit doesn’t break this centuries-old tradition.

The Transit of Venus

Before exploring the role of Venus transits in history, it’s worth taking a couple of steps back. It’s worth looking at the geometry of our Solar System to understand why this event is so rare.

Venus takes about 225 days to make one full orbit around the Sun while the Earth takes about 365 days. The two planets line up roughly once every year and a half; Venus lies directly between the Earth and the Sun. But we don’t see a transit every time because Venus’ orbit is tilted by about three degrees compared to Earth’s. From our perspective, we see Venus passing near the Sun on these occasions but not crossing it. Transits occur when the Earth and Venus line up at the same inclination of their orbits. That’s when we see the planet as a small dot crossing the Sun, and it’s a much rarer occurrence. Venus transits come in pairs eight years apart, but pairs come less than once per century. The repeating pattern between transits is eight years, 105.5 years, eight years, and 120.5 years.

But astronomers didn’t always know the transit schedule. In fact, they didn’t know nearly as much about planetary orbits as we know now. Getting a sense of where astronomy was as a science before transits became a valuable tool for astronomers is also worthwhile before getting into the story of transits in history.

Where We Stood

Until 1543, we were the centre of the Universe. Aristotelean and Ptolemaic models of cosmos had the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn orbiting around the Earth against the background of fixed stars. But astronomers observed odd behaviour like planets occasionally doubling back on their orbits that couldn’t be explained in this geocentric model. Polish astronomer Nicolaus Copernicus proposed an elegant, and controversial, solution. He decentered the Earth and posited that all planets, including the Earth, orbit the Sun. In this model, the odd planetary motions astronomers saw could be chalked up to their orbiting viewpoint. Copernicus published his model the year of his death, 1543, in his De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres). Though he didn’t see it, he changed the cosmic world view to one with a heliocentric system.

German astronomer Johannes Kepler built on Copernicus’ heliocentric model. Copernicus had retained the ancient idea that planets orbit the sun in perfect circles, but again the observations were inconsistent with the model. Kepler found that the planets actually trace elliptical orbits around the Sun, a theory he proved by using his model to accurately predict the November 7, 1631 transit of Mercury. In 1627, he also predicted the 1631 transit of Venus.

The 1631 Venus transit wasn’t visible in Europe, and Kepler, who died in 1630, failed to this transit’s pair. He predicted a Venus transit in 1761 and a near transit in 1639. He was wrong, and English astronomer Jeremiah Horrocks found the error and used Kepler’s adjusted calculation to predict the 1639 event. At around quarter past three on the afternoon of December 4 that year, he became one of the first men in history to observe a Venus transit. He projected the sun onto a piece of paper through a telescope. His friend William Crabtree also watched the event. Horrocks used his observations to guess at Venus’ size and compared data with Crabtree to estimate the distance between the Earth and the Sun.

From the Earth to the Sun

The actual distance between the Earth and the Sun eluded astronomers in the 17th century. By the 1660s, the Copernican heliocentric model was widely accepted and the planets’ relative orbits were well known. The missing piece was a number. Everything was quantified by the valueless Astronomical Unit (AU) where 1 AU is the average distance from the Sun to the Earth. Venus was known to orbit on average 0.7 AU from the Sun, but that wasn’t the precise value astronomers wanted. If they could determine the value for 1 AU, they could figure out the size of every planet’s orbit and the picture of the solar system, at least as it was understood at the time, would be complete.

Edmund Halley of Halley’s Comet fame was the first astronomer to come up with a way of using the transit of Venus to find the value for 1 AU. If two astronomers observed the transit from two far apart locations on Earth, they could use the difference in transit time and their known distance from each other to calculate the distance between the Earth and Venus. Then, applying Kepler's third law about the shape of planetary orbits – the square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit – they could determine the value of 1 AU.

French astronomer Joseph-Nicolas Delisle improved on Halley's method. He stipulated that if the two observers knew their exact positions on Earth, they would only need to record the moment when the edge of Venus lined up with the edge of the Sun. This would be enough to calculate the value of 1 AU.

Measuring the Solar System with Transits

Halley died in 1742, 19 years before he could try his method on the 1761 transit. But a host of astronomers took up the challenge in his stead. European expeditions set out to India, the East Indies, Siberia, Norway, Newfoundland, and Madagascar to get the best and most spaced out views of the event. From the whole worldwide network, more than 120 transit observations were recorded, but most were of poor quality stemming from optical problems and inexperienced observers. For the 1769 transit, more than 150 observations were recorded from Canada, Norway, California, Russia, and famously Tahiti as part of Captain James Cook’s first expedition. But the results were only marginally better.

The state of technology in the 17th century made it impossible to record the exact moments of the start and end of the transit because of the so-called black drop effect. As Venus crossing in front of the Sun, a haze obscured the planet making it impossible for astronomers to make clear observations. But even poor results are results. In 1771, French astronomer Jérôme Lalande combined the observations from the 1761 and 1769 transits and calculated that 1 AU was 95 million miles (153 kilometers) give or take a half million or so miles. It was a start, but it wasn’t the precise value astronomers had hoped for.

Over a century later, a new generation of astronomers sought to use the 1874 and 1882 pair of Venus transits to refine the value of 1 AU. This time around, reigning astronomical superpowers France and England weren’t the only nations mounting expeditions for the event. Austria, Belgium, Brazil, Denmark, Germany, Italy, Mexico, the Netherlands, Portugal, Russia, and the United States all joined in the international effort, though it was far from the organized enterprise we see in international cooperatives today.

A new technology was also on hand for this set of 19th century transits: photography. Most astronomers felt their photographic recording wasn’t good enough to provide accurate measurements. Only the American astronomers felt the 200 photographs they took during the 1874 transit were promising enough to try again in 1882.

The 1882 transit was visible in the United States, and the U.S. Naval Observatory produced nearly 1,400 photographs. Though a striking record, these and other images gathered from other sites around the world did little to perfect the standing value of 1 AU. American astronomer William Harkness studied the 1874 and 1882 photographs and came up with a value of 92,797,000 miles (149,342,295) give or take 59,700 miles for 1 AU. This was better, but it still wasn’t accurate enough. The black drop effect remained; perfect Earth-based observations can never be free from the distorting effects of the atmosphere.

New Technologies, New Goals

Space age technology made short work of the quest to find the value of 1 AU. Radio telemetry from space probes and radar measurements have yielded the value of 92,955,807.273 miles (149,597,870.700 kilometres), give or take about 100 feet. But just because this one big question has been answered doesn’t mean the 2004 and 2012 transits have to break the tradition of astronomers using the event to further our understanding of the Universe around us. This generation just has a very different goal in mind. Instead of measuring our Solar System, this pair of transits is helping astronomers measure the atmospheres of exoplanets.

2004 was the first transit since quantitative astronomical spectroscopy was invented, and astronomers took the opportunity to make detailed spectroscopic measurements of Venus’ upper atmosphere. Spectroscopy, which came onto the astronomical scene in the first half of the 20th century, allows astronomers to determine the chemical composition of a planet’s atmosphere. As sunlight passed through Venus’ atmosphere, the gases absorbed light at certain known wavelengths. The light that reached Earth had an absorption spectrum that astronomers read to find exactly what makes up the planet’s atmosphere.

Learning more about Venus wasn’t the only reason to decipher its atmosphere in 2004. Taking spectroscopic measurements was a practice run for applying the same method to determining the atmospheric composition of exoplanets – planets that orbit stars other than the Sun. Astronomers are using this 2012 transit to test another method of studying exoplanets.

Hubble will use its advanced Camera for Surveys, Wide Field Camera 3, and Space Telescope Imaging Spectrograph to view the transit in a range of wavelengths and perform spectroscopic analysis. But because its cameras are too sensitive to point directly at the Sun, Hubble will measure the light passing through Venus’ atmosphere as it reflects off the Moon. If Hubble can get an accurate reading of Venus this way, it will be another tool in astronomers’ arsenal for determining the atmospheric composition of exoplanets. If there’s an Earth out there, this could be the way to find it.

Over the course of astronomy’s history, Venus transits have shaped and given size to our Solar System. Now, transits are helping us understand our place in the Universe relative not only to other planets and stars but to other possible worlds and life forms. As you watch a small dot cross in front of a circle later, try to keep in mind the significance of and rich history behind this seemingly tiny event.